Design, characterisation and in vivo testing of a new, adjustable stiffness, external fixator for the rat femur

Abstract

Very little is known about the infl uence of the mechanical environment on the healing of large segmental defects. This partly refl ects the lack of standardised, well characterised technologies to enable such studies. Here we report the design, construction and characterisation of a novel external fi xator for use in conjunction with rat femoral defects. This device not only imposes a predetermined axial stiffness on the lesion, but also enables the stiffness to be changed during the healing process. The main frame of the fi xator consists of polyethylethylketone with titanium alloy mounting pins. The stiffness of the fi xator is determined by interchangeable connection elements of different thicknesses. Fixators were shown to stabilise 5 mm femoral defects in rats in vivo for at least 8 weeks during unrestricted cage activity. No distortion or infections, including pin infections, were noted. The healing process was simulated in vitro by inserting into a 5 mm femoral defect, materials whose Young's moduli approximated those of the different tissues present in regenerating bone. These studies confi rmed that, although the external fixator is the major determinant of axial stiffness during the early phase of healing, the regenerate within the lesion subsequently dominates this property. There is much clinical interest in altering the mechanics of the defect to enhance bone healing. Our data suggest that, if alteration of the mechanical environment is to be used to modulate the healing of large segmental defects, this needs to be performed before the tissue properties become dominant.

A Concert between Biology and Biomechanics: The Influence of the Mechanical Environment on Bone Healing

“…Results from these studies are not surprising and are consistent with the literature suggesting that extremely rigid fixation is detrimental to bone healing (Chao et al, 1989), and allowing increased flexibility during the later stages of healing is beneficial. Furthermore, this is attributed to the tissues occupying the fracture gap as was demonstrated by both Gardner et al (1996) and Glatt et al (2012a). They demonstrated if there is no material present in the fracture gap, which simulates conditions immediately post-surgery, the fixator frame provided stability at the fracture site with no contribution from the gap itself to support the fracture site.…”

Improved Healing of Large Segmental Defects in the Rat Femur by Reverse Dynamization in the Presence of Bone Morphogenetic Protein-2

“…Pilot Study E xternal fixators of three different stiffnesses were constructed as described previously 21 and used in a pilot study to determine their influence on the first two weeks of bone-healing in the presence of BMP-2. This study had two aims: (1) to test our hypothesis that the fixator with the least stiffness would promote the most rapid early healing response and (2) to determine a suitable time for increasing fixator stiffness.…”

“…Defects were stabilized with custom-made external fixators, as described previously 21 . Their key features are interchangeable connection elements of different stiffnesses.…”

“…T 21,2012 to predict an object's ability to resist torsion. All experimental groups had polar moment of inertia values greater than those of the control femur.…”

Preclinical testing of drug delivery systems to bone

“…A PEEK-based external fixator is commercially available for stabilizing rat long bones. This external fixator can be adjusted for stiffness [183]. Therefore, it is very suitable for research questions involving biomechanics in association with drug delivery systems.…”